1. Field of the Invention
The present invention relates to a charge-up suppressing film being hard to be charged, having less charges, and/or being capable of rapidly suppressing charges, and to a member constituting the charge-up suppressing film. The invention also relates to an electron beam apparatus and an image forming apparatus using the member.
2. Related Background Art
Two types of electron-emitting devices are known which are roughly classified into a thermal electron-emitting device and a cold cathode electron-emitting device. The types of a cold cathode electron-emitting device include a surface conduction emitting type, a field emission type (hereinafter called an FE type), a metal/insulator/metal type (hereinafter called an MIM type), and the like.
Examples of the surface conduction electron-emitting device are disclosed in Radio Eng. Electron Phys., by M. I. Elinson, 10, 1290 (1965) and other papers. The surface conduction electron-emitting device utilizes the phenomenon that when current is flowed in a thin film having a small area formed on a substrate in a direction parallel to the film surface, electron emission occurs. Reported thin films for a surface conduction electron-emitting device include an SnO2 thin film by Elinson et al, an Au thin film (xe2x80x9cThin Solid Filmsxe2x80x9d, by G. Ditter, 9, 317 (1972)), an In2O3/SnO2 thin film (xe2x80x9cIEEE Trans. ED Conf.xe2x80x9d, by M. Hartwell and C. G. Fonstad, 519 (1975)), a carbon thin film (xe2x80x9cVacuumxe2x80x9d, by Hisashi ARAKI, et al. vol. 26, No. 1. p. 22 (1983)), and the like.
As a typical example of a surface conduction electron-emitting device, the structure of an element proposed by M. Hartwell is schematically shown in FIG. 20. In FIG. 20, reference numeral 3001 represents a substrate, and reference numeral 3004 represents an electroconductive thin film which is made of a metal oxide thin film having an H-character shape formed through sputtering. An electron-emitting region 3005 is formed in the electroconductive thin film by an energization operation called an energization forming operation to be described later. A distance L is set to 0.5 to 1 mm, and a width W is set to 0.1 mm. The electron-emitting region 3005 is shown in the center of the electroconductive thin film 3004 as having a rectangular shape. These shape and position are schematically shown by way of example only for the convenience of drawing the device structure, and do not show actual shape and position.
Conventionally, the electron-emitting region 3005 of a surface conduction electron-emitting device is generally formed in the electroconductive thin film 3004 by the energization operation called the energization forming operation, so as to enable electron emission. With the energization forming operation, a d.c. voltage or a voltage rising very gently, e.g., at about 1 V/min, is applied across opposite ends of the electroconductive thin film 3004 to locally break, deform, or decompose the film 3004 to from the electron-emitting region 3005 having a high electric resistance. Fissures are formed partially in the electroconductive thin film 3004 locally broken, deformed, or decomposed. Electrons are emitted from the fissures and nearby areas when a proper voltage is applied to the electroconductive thin film 3004 after the energization forming operation.
Examples of the FE type are disclosed in xe2x80x9cField emissionxe2x80x9d, by W. P. Dyke and W. W. Dolan, Advance in Electron Physics, 8, 89 (1956), xe2x80x9cPhysical properties of thin-film field emission cathodes with molybdenum conesxe2x80x9d, by C. A. Spindt, J. Appl. Phys., 47, 5248 (1976) and other papers.
A typical example of the structure of the FE type device proposed by C. A. Spindt, et al is shown in the cross sectional view of FIG. 21. In FIG. 21, reference numeral 3010 represents a substrate, reference numeral 3011 represents an emitter wiring made of electroconductive material, reference numeral 3012 represents an emitter cone, reference numeral 3013 represents an insulating layer, and reference numeral 3014 represents a gate electrode. As a proper voltage is applied between the emitter cone 3012 and gate electrode 3014 of this device, electrons are emitted from the tip of the emitter cone 3012. Another structure of the FE type device has the emitter and gate electrode disposed on the substrate generally in parallel to the substrate surface, as different from the lamination structure shown in FIG. 21.
Examples of the MIM type are disclosed in xe2x80x9cOperation of Tunnel-Emission Devicesxe2x80x9d, by C. A. Mead, J. Appl., Phys., 32, 646 (1961) and other papers. A typical example of the MIM type device structure is shown in the cross sectional view of FIG. 22. In FIG. 22, reference numeral 3020 represents a substrate, reference numeral 3021 represents a lower electrode made of metal, reference numeral 3022 represents an insulating film as thin as about 100 angstroms, and reference numeral 3023 represents an upper electrode made of metal and having a thickness of about 80 to 300 angstroms. When a proper voltage is applied across the upper electrode 3023 and lower electrode 3021 of the MIM type device, electrons are emitted from the surface of the upper electrode 3023.
As compared to a hot cathode electron-emitting device, a cold cathode electron-emitting device can emit electrons at a lower temperature so that a heater is not necessary. Therefore, the structure of the cold cathode electron-emitting device is simple and a small device can be manufactured. Furthermore, even if a number of devices are mounted on a substrate at a high integration, a problem that the substrate is heated and melted is not likely to occur. As different from a slow response time of the hot cathode electron-emitting device because it operates by heating the cathode with a heater, the cold cathode electron-emitting device has a fast response time.
From the above reasons, researches are widely conducted in order to use cold cathode electron-emitting devices in various fields.
For example, since the structure of a surface conduction electron-emitting device among cold cathode electron-emitting devices is simple and the manufacture thereof is easy, a number of elements can be disposed in a large area. A method of driving a number of devices disposed on a substrate has been studied, for example, as disclosed in Japanese Patent Application Laid-open No. 64-31332.
As applications of surface conduction electron-emitting devices to an image forming apparatus such as an image display apparatus and an image forming apparatus, a charged beam source and the like have been studied. An application to the image display apparatus has been studied which uses a combination of surface conduction electron-emitting devices and fluorescent members radiating light upon application of an electron beam, as disclosed in U.S. Pat. No. 5,066,883, Japanese Patent Application Laid-open Nos. 2-257551 and 4-28137. The image forming apparatus using a combination of surface conduction electron-emitting devices and fluorescent members has excellent characteristics expected more than other types of conventional image forming apparatuses. For example, as compared to recently prevailing liquid crystal display apparatuses, the image forming apparatus is of a self light emission type so that it has advantages such as no back light and a broad view angle.
A method of driving a number of FE type devices disposed on a substrate is disclosed, for example, in U.S. Pat. No. 4,904,895 assigned to the present assignee. An example of application of FE type devices to an image forming apparatus is a flat panel display apparatus reported, for example, in xe2x80x9cRecent Development on Microtips Display at LETIxe2x80x9d, by R. Meyer, Tech. Digest of 4th Int. Vacuum Micro electronics Conf., Nagahama, pp. 6 to 9 (1981).
An example of an image forming apparatus using a number of MIM type devices is disclosed, for example, in Japanese Patent Application Laid-open No. 3-55738.
Of the image forming apparatuses using electron-emitting devices, a thin, flat panel type display apparatus is light in weight and does not require a large installation space. The flat panel display apparatus has drawn attention as can be replaced by a CRT display apparatus.
FIG. 23 is a perspective view showing an example of a display panel of a flat type image forming apparatus and partially broken to show the internal structure thereof.
In FIG. 23, reference numeral 3115 represents a rear plate, reference numeral 3116 represents a side wall, and reference numeral 3117 represents a face plate. The rear plate 3115, side wall 3116, and face plate 3117 constitute an envelope (hermetically sealed container) which maintains the vacuum state of the inside of the display panel.
A substrate 3111 is fixed to the rear plate 3115 and formed with nxc3x97m cold cathode devices (n and m are a positive integer of 2 or larger and are properly set in accordance with the number of necessary pixels). As shown in FIG. 23, the nxc3x97m cold cathode devices 3112 are wired to m row-directional wiring patterns 3113 and n column-directional wiring patterns 3114. The structure constituted of the substrate 3111, cold cathode devices 3112, row- and column-directional wiring patterns 3113 and 3114 are called a multi-electron beam source. An insulating layer (not shown) is formed at least in the cross area between the row- and column-directional wiring patterns to electrically insulate the wiring patterns.
The inner surface of the face plate 3117 is formed with a fluorescent film 3118 made of fluorescent materials of red (R), green (G), and blue (B) primary colors. A black body (not shown) is formed between fluorescent materials of each color of the fluorescent film 3118. A metal back 3119 made of Al is formed on the fluorescent film 3118 on the side of the rear plate 3115.
Dx1 to Dxm and Dy1 to Dyn represent connection terminals of an air tight structure for electrically connecting the display panel to an unrepresented external electronic circuit. Dx1 to Dxm are electrically connected to the row-directional wiring patterns 3113 of the multi electron beam source, Dy1 to Dyn are electrically connected to the column-directional wiring patterns 3114 of the multi electron beam source, and Hv is electrically connected to the metal back 3119.
The inside of the air-tight envelope is maintained in a vacuum state of about 10xe2x88x926 Torr. As the display area of the image forming apparatus becomes large, a pressure difference between the inside and outside of the air-tight envelope becomes large. Therefore, it is necessary to provide means for preventing deformation or breakage of the rear plate 3115 and face plate 3117. If the rear plate 3115 and face plate 3117 are made thick, not only the weight of the image forming apparatus increases but also distortion and parallax of an image appear when the image is viewed obliquely. In the example shown in FIG. 23, a structure support (called a spacer or rib) 3120 made of a relatively thin glass plate is provided for supporting the atmospheric pressure. A space between the substrate 3111 formed with the multi electron beam source and the face plate 3116 formed with the fluorescent film 3118 is generally maintained in the order of sub-millimeter to several millimeters and the inside of the air-tight envelope is maintained in a high vacuum state.
With the image forming apparatus using the display panel described above, when a voltage is applied to each cold cathode device 3112 via the terminals Dx1 to Dxm and Dy1 to Dyn, electrons are emitted from the cold cathode device 3112. At the same time, a high voltage of several hundred V to several kV is applied to the terminal Hv to accelerate the emitted electrons and collide them with the inner surface of the face plate 3117, so that the fluorescent material of each color of the fluorescent film 3118 is excited to radiate light and display an image.
The display panel of the image forming apparatus described above is associated with the following problems.
Firstly, the spacer 3120 is possibly charged because some electrons emitted near the spacer 3120 collide with the spacer 3120 or because of a reaction of electron emission. The trajectory of electrons emitted from the cold cathode device 3112 may be deflected by the charged spacer and the electrons reach positions different from normal positions on the fluorescent film. Therefore, an image near the spacer is distorted.
Secondly, in order to accelerate electrons emitted from the cold cathode devices 3112, a high voltage of several hundred V or higher (i.e., high electric field of 1 kV/mm or higher) is applied between the multi beam electron source and the face plate 3117. There is a fear of creeping discharge on the surface of the spacer 3120. If the spacer is charged, this creeping discharge is possibly induced.
In order to solve the above problems, it has been proposed to flow a small current through the spacer to eliminate charges (Japanese Patent Application Laid-open Nos. 57-118356, 61-124031). In this proposal, a small current is flowed through the spacer surface by forming a high resistance thin film on the insulating spacer surface. This charge-up suppressing film is made of a tin oxide film, a mixed crystal thin film of tin oxide and indium oxide, or a metal film.
A semiconductor thin film such as tin oxide used in this proposal is very sensitive to gas such as oxygen to the degree that it is used for a gas sensor. Therefore, this thin film is likely to change its resistance value with the atmosphere. The above materials and metal film have a low resistivity so that a film is formed in an island shape or very thin in order to obtain a high resistance value. A conventional high resistance film is therefore associated with difficult reproductivity and is likely to change its resistance by a thermal process such as frit sealing and baking during the display assembly process.
Furthermore, the amount of secondary electrons generated when some electrons emitted near the spacer 3120 collide with the high resistance film depends on the conditions and thickness of the high resistance film. Therefore, there is a variation in the degree of removing charges in the in-plane of the high resistance film formed in an island shape or very thin.
The present application discloses the invention which can solve the problems of a conventional spacer and provide a high reliability charge-up suppressing film for a spacer and an image forming apparatus using the charge-up suppressing film.
The charge-up suppressing film of this invention is configured as in the following.
The charge-up suppressing film comprises an electroconductive first film and a second film formed on the first film to partially expose the first film.
Specifically, it is preferable that the surface has a sufficiently large number of unevenness as viewed in a cross section taken along a normal to the charge-up suppressing film. In this invention, the second film is partially formed and the first film is partially exposed so that charges are difficult to be generated. Since charges are difficult to be generated and the first film is electroconductive, charges can be removed easily.
The secondary electron emission coefficient of the second film is preferably smaller than that of the partially exposed first film.
The second film may be formed on the first film in an island shape.
The second film may be formed on the first film in a dispersed manner.
The first film may have a thickness of 10 nm to 1 xcexcm. The second film may have a thickness of 1 nm to 10 nm.
Various materials can be used for the second film. For example, the material may be carbon or electroconductive particles. The conductivity of the second film may be lower than that of the first film.
The member of the invention is configured as in the following.
A member comprises a first electroconductive member and a second member formed on the first member to partially expose the first member.
The first member may be formed on a substrate, and the substrate may be electrically insulative. The first member may be a film formed on the substrate.
An electron beam apparatus of this invention is configured as in the following.
An electron beam apparatus comprises an electron source, a radiative member on which electrons emitted from the electron source is radiated, and a third member provided between the electron source and the radiative member, the third member comprising an electroconductive first member and a second member formed on the first member to partially expose the first member.
The third member may be a support member for supporting a distance between the electron source or a first substrate formed with the electron source and the radiative member or a second substrate formed with the radiative member. Specifically, the third member is a spacer or an outer frame.
An image forming apparatus of the invention is configured as in the following.
An image forming apparatus comprises an electron source, an image forming member for forming an image based on electrons emitted from the electron source, and a third member provided between the electron source and the image forming member, the third member comprising an electroconductive first member and a second member formed on the first member to partially expose the first member.